The average weekday circulation of U.S. daily newspapers fell 2.6 percent in the six months that ended in September, led by a 16.6 percent drop for The Chronicle.

The figures released Monday by the Audit Bureau of Circulation, which certifies the accuracy of sales figures, continues a long-term trend of flat to sinking readership.

The average Sunday circulation for U.S. dailies fell 3.1 percent, compared with September 2004, according to an analysis by the Newspaper Association of America.

The Chronicle said its paid Sunday circulation declined 13.5 percent to 467,216 copies. On Monday through Saturday, it averaged sales of 400,906 copies during the six-month period.

Chronicle Publisher Frank Vega said the newspaper's circulation declines -- nearly 80,000 weekday papers and just over 73,000 on Sunday -- were owed almost entirely to a deliberate strategy of trimming promotional distributions and giveaways.

"We cut a lot of what you would call unprofitable circulation around the first of the year," Vega said. "We made a decision that we want quality, profitable circulation that better serves our advertisers."

David Cole, editor of NewsInc., a publication that tracks the newspaper industry, said other newspapers, notably the Des Moines Register, have taken similar steps in recent years. Cole said the best business reason for a newspaper to cut its circulation is to save on printing and distribution.

The Chronicle, which is privately owned by the Hearst Corp. and does not disclose its finances, reportedly lost more than $60 million in 2004. Vega declined to discuss the paper's finances.

Among the nation's largest newspapers, the Atlanta Journal-Constitution and the Boston Globe each reported weekday circulation declines in excess of 8 percent.

Among other Bay Area papers, Sunday circulation at the San Jose Mercury News dropped 5.25 percent to 278,420; weekday sales fell 3.9 percent to 249,090. The Mercury News is owned by Knight Ridder, the publicly traded newspaper chain that is under pressure from large shareholders dissatisfied with its stock performance.

Sunday circulation at the Oakland Tribune rose 2.9 percent to 67,613, according to Monday's report, but Monday-through-Friday sales fell 3.4 percent to 66,811.

The Contra Costa Times bucked the trend with a gain of 137 subscribers that brought its Sunday circulation up to 193,778. Weekday sales rose by 186 to 182,834.

John Kimball, chief marketing officer for the Newspaper Association of America, blamed the industrywide circulation drop on forces ranging from increased Internet competition to limits on telemarketing calls, which many papers had used to sell subscriptions.

"What we're seeing is a continuation, and frankly an acceleration, of a trend," Kimball said, adding that The Chronicle is particularly vulnerable to Internet competition given that it is in "one of the most wired markets in America."

Newspapers have sought to blunt the significance of circulation drops by emphasizing a different number, called readership, that takes into account the fact that a single copy may be read by more than one person.

Indeed, Chronicle spokeswoman Patty Hoyt said Monday that the paper's readership -- as measured by the nationally recognized Scarborough Research -- remained just above 1 million on weekdays and was nearly 1.2 million on Sundays.

But, Rick Colsky, who runs an agency in San Francisco that buys advertising space, said ad purchasers s still tend to rely on circulation "because the readership is too iffy."